1. Field of the Invention
The present invention relates to an anodic oxide (alumite) film formed on the surface of aluminum or aluminum alloy and an anodizing method for obtaining such a film. In particular, the present invention relates to an anodic oxide film with excellent corrosion resistance and uniform film thickness and an anodizing method for obtaining such a film.
2. Description of the Related Art
Members having an anodic oxide film formed on the surface of aluminum or aluminum alloy have been used conventionally for automotive parts and the like used under a corrosive condition. Aluminum parts such as outboard motors which require high corrosion resistance have to be subjected to a rust-preventive treatment after an anodizing (alumite) treatment in order to prevent the spread of rust into a mother material through pores generated in honeycomb cells of the anodic oxide film.
For example, a post-anodization sealing for improving the corrosion resistance is described in Japanese Patent Application Examined Publication No. 7-103479/1995. In particular, according to conventional direct current anodization by using an apparatus shown in FIG. 15, cells grow linearly so that pores generated in the cells tend to grow linearly. Accordingly, water which will be a cause for rust tends to invade into the pores. To prevent the water invasion, a rust-preventive treatment such as the above-described sealing is necessary.
When a groove or like of the first piston ring of an engine is anodized for improving its abrasion resistance, it is difficult to have uniform growth of an anodic oxide film because aluminum or aluminum alloy used for the piston or the like has been segregated owing to a large silicon content (8% by weight or greater but not greater than 30% by weight). As a result, the film thickness is not uniform and varies widely. Not limiting to the silicon, an impurity and/or additive, when incorporated in aluminum or aluminum alloy, tends to inhibit the growth of an anodic oxide film around the impurity and/or additive. It is because honeycomb cells of the anodic oxide film grow in one direction (perpendicular to the surface of a mother material) so that in the sites where the impurity and/or additive such as silicon precipitates heavily, the impurity and/or additive prevents the anodic oxide film from growing. Consequently, it is difficult for the anodic oxide to grow.
As a measure for overcoming the above problem, for example, Japanese Patent Application Unexamined Publication No. 5-17899/1993 discloses a method of forming a flat anodic oxide (alumite) film on a substrate comprising steps of re-melting Si particles precipitated in the vicinity of the substrate surface by exposure to a high-density energy heat ray from a TIG welder so that the Si particles will be miniaturized or diluted; and anodizing the substrate. This method, however, requires the re-melting step in addition to the anodizing step and may be accompanied with another problem such as thermal strain.
In Japanese Patent Application Unexamined Publication No. 10-237693/1998, described is a method of mechanically crushing an anodic oxide film, which has grown unevenly owing to the inhibition of the film growth by silicon, by roller-burnishing, thereby obtaining a film with a flat surface. Since this method requires, in addition to the anodizing step, a roller-burnishing step, it cannot be applied to minute places such as a groove of a piston ring which may be broken by roller-burnishing.
In Japanese Patent Application Unexamined Publication No. 2001-271704, described is a method comprising steps of anodizing in an aqueous solution of phosphoric acid and a fluoride to obtain a flat surface, immersing the resulting minute pores with a thermosetting resin and curing the thermosetting resin. This method, however, requires two steps in addition to the anodizing step.
On the other hand, it is known that an anodic oxide film having improved impact resistance can be obtained by incorporating silicon in the anodic oxide film. In the prior art, however, the growth of an anodic oxide film is disturbed around silicon and it is therefore difficult to form an anodic oxide film while the mother material comprises silicon.
As prior art for overcoming the above problem, described in Japanese Patent Application Unexamined Publication No. 2002-89361 is a method of washing an aluminum surface with a strong alkali solution prior to anodization so as to dissolve the aluminum surface for causing silicon to expose from the surface while adjusting the exposure amount in accordance with a height of the anodic oxide (aluminum) film around the exposed portion expanded by oxidation, thereby obtaining a flat surface. This method, however, requires a washing step with a strong alkali in addition to the anodizing step.
In Japanese Utility Model Application Unexamined Publication No. 6-14620/1994, disclosed is a method comprising steps of re-melting Si particles, which have been precipitated in the vicinity of the substrate surface, by exposure to energy beam heat ray such as electron beam so as to miniaturize and uniformly disperse the Si particles, and then anodizing the surface. According to this method, an anodic oxide film can be formed while incorporating silicon therein. However, the method requires a re-melting step in addition to the anodizing step and may therefore be accompanied with the problem such as thermal strain.
When anodization is adopted for rust prevention of an aluminum part such as outboard motors, formation of a relatively thick anodic oxide film is required for obtaining high rust preventing effects. Then, a longer anodization period is required to obtain a film having a desired thickness so that difficulty is encountered in reducing anodization time.